DESCRIPTION (application abstract): "Troponin T (TnT) is one of three proteins (including TnI and TnC) that form the troponin complex (Tn), which regulates skeletal muscle contraction. Sequence analysis of the TnT gene predicts as many as 64 different isoforms, due to the presence of 5 alternatively spliced exons at the N-terminus and 2 mutually exclusive alternatively spliced exons (alpha and beta) at the C- terminus. Previous studies have suggested that the role of TnT in the regulation of contraction is to interact with and anchor the complex of TnI and TnC to actin-containing filaments through TnT's interaction with tropomyosin (Tm). Two recent results from our lab suggest that, in addition to this role, TnT is critically involved in the regulation of actomyosin ATPase activation. First, the C-terminus of TnT (containing alpha or beta) interacts with TnC, and this interaction seems to be required for the activation of actomyosin ATPase activity. This interaction in part determines the Ca2+ -sensitivity of contraction, which differs depending on whether alpha or beta is present. Second, the N-terminal region of TnT (containing various alternatively spliced exons) appears to determine the level of activation of actomyosin ATPase activity once this interaction with TnC occurs. Our recent observations implicate both alternatively spliced regions (N- and C-) of TnT in the regulation of actomyosin ATPase activation. The major question being addressed in this proposal is: how do structural changes in TnT, conferred by alternative splicing in these two regions, affect this novel function of TnT? Our working hypothesis, which will be tested in the present proposal, is that Ca2+ binding to TnC causes an interaction between TnC and the C-terminus of TnT (containing alpha or beta) and causes an activation of the ATPase activity. The Ca2+ -sensitivity of this activation depends on the alpha/beta splice variants and the maximum level of activation is determined by the particular N-terminal TnT variant which is present."